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The free update fixes a vulnerability in the way PDF files handle fonts, where a malicious PDF file could sneak malware into your iOS device, giving hackers access to your hardware. However, that same vulnerability allows easy jailbreaking of iOS 4.3.3 with the web-based JailBreakMe 3.0, which frees users from the tight restrictions Apple places on its iOS software. Apple certainly wanted to stop that as quickly as possible.
I downloaded the update, and because of Apple's cumbersome process requiring users to download the entire operating system file rather than an incremental update (which will be fixed in iOS 5), the upgrade took a seemingly interminable 25 minutes from start to finish on my two-year-old Windows laptop:
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